All living organisms, even plants, require oxygen to survive and thrive. A low oxygen concentration will cause the aquariums inhabitants to slowly suffocate making them increasingly likely to become ill. Oxygen is also required by an aquariums biological filtration to convert toxic ammonia and nitrite to nitrate. The Tetra Test enables you to easily measure oxygen levels in your aquarium to ensure that you are providing suitable conditions. An easy guide and comparison chart is included. Each test kits provides around 30 tests.